Spectroscopy is a general term for the process of measuring energy or intensity as a function of wavelength in a beam of electromagnetic radiation (e.g., light). Many conventional spectrometers include basic features and components such as a slit and a collimator for producing a parallel beam of radiation, one or more prisms or gratings for dispersing radiation through differing angles of deviation based on wavelength, and apparatus for collecting and measuring characteristics of dispersed radiation. Spectroscopy uses absorption, emission, or scattering of electromagnetic radiation by molecules or ions to qualitatively and quantitatively study physical properties and processes of matter.
Light or radiation directed at a target, or sample of physical matter, during operation of a spectrometer system may be referred to as incident radiation. Redirection of incident radiation following contact with a sample commonly is referred to as scattering of radiation. To the extent that atoms or molecules in a sample absorb all or a portion of incident radiation, rather than reflect incident radiation, a sample may become excited, and the energy level of the sample may be increased to a higher energy level. Electromagnetic radiation that passes through a sample may produce a small portion of light that is scattered in a variety of directions. Light that is scattered but continues to have the same wavelength as the incident radiation will also have the same energy, a condition often referred to as Rayleigh or elastically scattered light. Incident radiation that is scattered during a change of vibrational state in molecules may be scattered with a different energy, and such scattered light may be called Raman scattered light. Such phenomena have been used in conjunction with spectroscopy to qualitatively and quantitatively study physical properties and processes, including identification of chemical properties, compositions, and structures of a sample.
A wave of electromagnetic radiation may be characterized by wavelength (the physical length of one complete oscillation) and by frequency of the wave (the number of oscillations per second that pass a given point). The wavelength of incident radiation on a sample may remain substantially unchanged in scattered radiation. Alternatively, the wavelength in scattered radiation may shift to one or more different wavelengths relative to the incident wavelength. The wavelength differential between the incident radiation and the scattered radiation may be referred to as a Raman shift. Spectroscopic measurement of Raman scattered light is a measure of the resulting wavelength of such scattered light.
Raman scattering may occur at wavelengths shifted from the incident light by quanta of molecular vibrations. The phenomenon of Raman scattered light, therefore, is useful in spectroscopy applications for studying qualities and quantities of physical properties and processes, including identification of chemical properties, compositions, and structures in a sample. Measurement of scattered radiation may enable identification of one or more frequencies associated with the sample, as well as the intensities of those shifted frequencies. The frequencies may be used to identify the chemical composition of a sample. If, for example, intensities are plotted on a Y-axis, and frequency or frequencies are plotted on an X-axis, the frequency or frequencies may be expressed as a wavenumber (the reciprocal of the wavelength expressed in centimeters). The X-axis, showing the frequency or frequencies, may be converted to a Raman shift in wavenumbers (a measure of the difference between the observed wavenumber positions of spectral bands) and the wavenumber of radiation appearing in the incident radiation.
Raman scattering offers a significant opportunity for qualitative and quantitative studies of physical properties and processes, including identification of chemical compositions and structure in samples of physical matter. However, Raman scattering is a comparatively weak effect when compared with Rayleigh or elastic scattering. Only about one scattered photon in about 106 to about 108 photons tends to be Raman shifted.
Detection limits in Raman spectroscopy are decreased by ambient light and background interference during sampling. Ambient light usually takes the form of interior lighting or sunlight, which can overpower even the strongest scattering samples. Thus, detectors and samples being scanned are typically fully enclosed to shield from ambient light. Samples that cannot be fully enclosed present special challenges.
Excitation sources used in Raman spectroscopy include gas lasers such as helium-neon, helium-cadmium, argon-ion, krypton-ion, as well as solid-state lasers including Nd-YAG, and diode lasers, solid-state tunable lasers, liquid dye lasers, fiber lasers, and other lasers.
Background interference also comes from non-spontaneous emissions from some types of samples, such as fluorescence. Fluorescence occurs when absorbed radiation is reduced in frequency by internal molecular processes and emitted as radiation that is closer to the red end of the visible light spectrum. Fluorescence sometimes may be strong enough in comparison with the Raman shift to swamp, or substantially overwhelm, the weaker Raman signal. Fluorescence decreases the dynamic range and ultimately the signal-to-noise ratio of data obtained from a sample. Fluorescence can be reduced by exciting at higher wavelengths, such as 1064 nm, but at the cost of expensive components and a loss of signal-to-noise ratios for all samples (i.e., even those samples not plagued by the problem of fluorescence). The loss of signal-to-noise is due to poor detectors at this wavelength and because Raman scattering varies with the wavelength to the negative fourth power (λ−4).